A variety of devices have been proposed for illuminating electronic displays and keypads. These devices include backlighting panels, front lighting panels, concentrators, reflectors, structured-surface films, and other optical devices for redirecting, collimating, distributing, or otherwise manipulating light. Passive optical components (for example, lenses, prisms, mirrors, and light extraction structures) are well-known and are used in optical systems to collect, distribute, or modify optical radiation.
Efficient use of light is particularly important in battery powered electronic displays and keypads such as those used in cell phones, personal digital assistants, and laptop computers. By improving lighting efficiency, battery life can be increased and/or battery sizes can be reduced. Prismatic films are commonly used to improve lighting efficiency and enhance the apparent brightness of a backlit liquid crystal display, and multiple light sources (for example, light emitting diodes (LEDs)) are commonly used for this purpose in keypads.
Lighting quality is also an important consideration in electronic displays and keypads. One measure of lighting quality for a backlit display or keypad is brightness uniformity. Because displays (and, to a somewhat lesser extent, keypads) are typically studied closely or used for extended periods of time, relatively small differences in the brightness can easily be perceived. These types of variances in brightness can be distracting or annoying to a user. To soften or mask non-uniformities, a light scattering element (for example, a diffuser) can sometimes be used. However, such scattering elements can negatively affect the overall brightness of a display or keypad.
Multiple light sources can alternatively be used to achieve brightness uniformity, but this approach has the associated disadvantage of reduced battery life. Thus, there has been some attention to the development of various means of effectively distributing the light from a more limited number of light sources, including the development of light guides comprising a plurality of light extraction structures. Such light extraction structures, as well as light extraction structure arrays, have been made by a number of different techniques, each having a different set of strengths and weaknesses.